In recent years, depletion of water resources has become serious, and use of water resources which have not hitherto been utilized is being studied. In particular, technology for producing drinking water from seawater which is most familiar and cannot be utilized as it is, so-called “seawater desalination”, is attracting attention. The seawater desalination has conventionally been put to practical use mainly by evaporation methods in the Middle East region which is extremely poor in water resources and very rich in thermal resources due to petroleum. However, in regions other than the Middle East, which are not rich in thermal resources, reverse osmosis methods have been adopted due to high energy efficiency, and many plants have been constructed and practically operated in such the Caribbean and the Mediterranean areas. Recently, improvement of reliability and cost reduction due to technical progress of the reverse osmosis methods proceed, and many seawater desalination plants utilizing reverse osmosis technology have been constructed also in the Middle East and have shown global expansion into Asia, Africa, Oceania and Central and South America.
It is preferred that the raw water subjected to the seawater desalination plant is taken from a place as far away from a coast as possible, from the viewpoint of its clarity and reduced environmental impact. In the case of a shoal, the length of an intake pipeline must be longer, so that cost problems tend to occur. When complicated like a bay, clear seawater suitable for the seawater desalination is not obtained in some cases even when the length of the intake pipeline is longer. For this reason, it is not uncommon to take the water from the vicinity of the coast. However, when there is a river in the vicinity of an intake point, fresh water from the river tends to flow in, and moreover, the ratio thereof largely fluctuates according to the ebb and flow of the tide, rain fall or the like. For example, a seawater desalination plant in Trinidad and Tobago is affected by a large river, and the salt concentration of raw water fluctuates from 1.5% to 3.5% (Non-Patent Document 1). The fluctuations in concentration of the raw water leads to fluctuation in osmotic pressure, that is to say, fluctuations in operating pressure, so that it has become necessary to appropriately adjust the output of a pump.
In order to fluctuate the output of the pump, attaching a flow rate regulation valve to a discharge portion of the pump or regulation with an inverter is commonly selected. However, the former cannot avoid energy loss due to pressure loss, although it is simple and low in cost. The latter leads to an increase in equipment cost, and it becomes a costly large restriction to attach the inverter to a large pump applied to a huge seawater desalination plant which has recently continued to increase, although it can suppress the energy loss.
Further, frequent fluctuations of operating pressure results in continuing to give stress to pipelines, valves and reverse osmosis membranes, and also results in accelerating fatigue of materials. This is therefore not a preferred situation.
Furthermore, fluctuations of raw water concentration results in fluctuations of product water quality. From the viewpoint of product water quality, it becomes necessary to perform designing based on the most severe case, so that the plant is overengineered in many cases. Methods for solving this include recirculating of concentrate in the case where the raw water concentration is low as shown in Patent Document 1 and recirculating of product water which does not satisfy the water quality as shown in Patent Document 2. However, circulation reprocessing leads to a decrease in product water amount and an increase in operating cost.
Measures which can reduce the changes in concentration include a method of installing a raw water storage tank. However, only the installation of the water storage tank results in relying on natural retention, and moreover, water flow is one-way from an inlet to an outlet. Accordingly, the concentration cannot be homogenized so much. When stirring is performed with the installation of the raw water storage tank, the concentration can be homogenized. However, this requires stirring energy, and is not practical in terms of energy and equipment, particularly in a huge plant.